1. Field of the Invention
The present invention relates to a cell data switching method and a cell data switching apparatus for switching cell data on an ATM (asynchronous transfer mode) information communication network to which computers, controllers, monitors, etc. are connected.
2. Description of the Related Art
Recently, in order to enhance control operations and human interface of monitor information in a process control system, there has been a demand for the advent of an environment in which ordinary physical control information, as well as multimedia data including sound and moving pictures, can be transmitted.
The process control system adopts an information communication network for handing data among computers, controllers, monitors, etc. In particular, in the field of process control in which high-speed data switching on the network is required, an information switching system using common memories (virtual shared memory) is adopted. In this system, each device can access data possessed by another device as if there were no transmission apparatus.
Each device is provided with a common memory with divided address areas corresponding to all respective devices. The device stores its own data in a specific area of its own common memory and sends out the data to the transmission path of the network at a predetermined cycle, thereby to transmit the data to the other devices. Each device receives data from the other devices through the transmission path of the network and stores the received data in the associated areas of its own common memory.
This transmission system is generally called "scan transmission system." In brief, each device receives data from the other devices through the transmission path and stores the received data in the associated area of its own common memory. In addition, each device sends out updated data from the specific area of its own common memory at a predetermined cycle. Thus, data is transferred among the respective devices. In this case, the updated data sent out from the specific area of the common memory of each device to the transmission path is called "scan data."
On the other hand, an ATM (asynchronous transfer mode) information communication network is known as a communication network suitable for transmitting multimedia data. There are a connection type ATM information communication network and a connectionless type ATM information communication network. In the network of either type, all data is transmitted in units of cell data having a fixed length of 53 bytes. Data is transmitted on the ATM information communication network by ATM switch apparatus having ATM switches.
If the ATM information communication network is used for transmission of process control data, an environment is created in which not only physical control information (process control data) but also multimedia data including sound and moving pictures can be transmitted.
However, if the process control data and other data (multimedia data including sound and moving pictures) are mixedly transmitted by using the conventional ATM information communication network, scan data cannot efficiently be scan-transmitted or multi-destination-transmitted among communication nodes provided with switching functions and arranged on the ATM information communication network.